The readily available grades in the UK are 303, 304 and 316. 303 is the cheapest, but least rust resistant and not weldable, 304 is similar but weldable, 316 is expensive, and toughest to work (because of the high level of Chrome), but is regarded as the food and marine grade since it is the most rust resistant and also weldable. If you're buying nuts and bolts there are two grades A2 and A4 which are broadly equivalent to 303 and 316 respectively.
If you are threading 316 don't even think about carbon steel dies, HSS only and preferably taken in stages, particularly if cutting large coarse threads - M8 not too bad, M20 best of luck - you'd really need to thread it in a lathe, finishing with a dienut or use a Coventry diehead that can be opened up to take the cuts in small increases in depth.
Nick